In UK patent application no. (ref. 2004P01248 GB) filed on the same day as the present application, hydraulic restrictors are used in an apparatus for damping the torsional excitation of a drive shaft. The damping is linear and viscous. Accordingly, it is required that the design of restrictor used promote laminar flow, and provide a constant and repeatable restriction (pressure-drop/flow characteristic) under all operating conditions.
In the case of a fixed value restrictor, laminar flow and constant and repeatable restriction may be achieved by careful design of the geometry of the restrictor (this is of course up to a certain flow rate threshold). In the case of a variable restrictor, the design is much more complex, and consequently stable/repeatable operation is difficult to achieve.